Japanese Patent Application No. 60066675 describes that when a converter is overloaded, the intermediate circuit voltage increases. As soon as the maximum allowable current or a higher current is output by the converter, an overload detector is activated and the DC voltage in the intermediate circuit is increased.
Japanese Patent Application No. 60066675 does not describe adapting the DC voltage to the supply voltage.
German Published Patent Application No. 30 21 119 describes a power inverter circuit for operating a variable-speed asynchronous motor. In this circuit, the frequency at which the asynchronous motor is controlled is increased to counteract the overvoltages that occur in the generator mode operation of the asynchronous motor or in the voltage supply system and the resulting excessively high currents.
This procedure has a disadvantage that, in increasing the frequency at which the asynchronous motor is operated, its speed increases. Thus speed regulation, which must be continuously available for many applications, can no longer be performed in the event of overvoltages.
German Patent No. 36 01 160 describes a control procedure for a frequency converter for starting, varying speed, braking, and positioning of three-phase asynchronous motors. The frequency converter has a three-phase or AC current converter and a smoothing capacitor as the intermediate circuit, and a downstream three-phase bridge circuit containing power transistors and an associated pulse width modulator to convert the intermediate circuit voltage into an AC voltage with adjustable frequency and amplitude. The transistors in the power inverter are blocked by a contactor circuit after the limit voltage is exceeded in the intermediate circuit. Thus, the power flow is interrupted in the power inverter and the voltage in the intermediate circuit is reduced due to the power consumption. After the intermediate circuit voltage has been reduced to less than the turn-off limit voltage, the power transistors are made conductive again. In the switch-off phase, the control frequency of the power transistors is considerably reduced and, when the power is turned on again, the power transistors are turned on at a lower frequency.
A disadvantage of this procedure is that when the supply voltage is excessively high, the power transistors are constantly turned on and off, which results in a constantly fluctuating output power of the motors connected. This non-continuous motor operation does not allow the operating conditions required by most applications to be achieved. Furthermore, high electromagnetic interference fields are generated by the constant switching of high currents, which require additional shielding; this represents a high level of stress on the power transistors, so that they break down more often.
German Patent No. 41 20 611 describes a method of limiting the intermediate circuit voltage of an intermediate voltage circuit converter that supplies a variable speed three-phase drive. It has an excitation limiter, which limits the current in the generator mode so that the intermediate circuit voltage does not exceed a certain value. Furthermore, an effective current setpoint value is defined through which the intermediate circuit voltage is kept at a sufficiently high value in the event of line failure. A higher-level speed controller defines the effective current setpoint value, which is blocked in the event of a line failure. In the generator mode, a first upper value and, in the event of a line failure, a second lower value are supplied to the excitation limiter as setpoint values for the intermediate circuit voltage.
A disadvantage of this method is that only in the generator mode and in the event of a line failure is the intermediate circuit voltage controlled. If an excessive line voltage occurs, no control measures are taken to ensure proper operation of the load connected to the converter.
German Patent No. 43 22 379 describes a circuit arrangement for protecting semiconductor switches when a line-controlled line power converter bridge, which feeds the recovered power back to the line, becomes conductive. This circuit arrangement makes it possible to short circuit the intermediate circuit-side output of the line power converter bridge in the event of an overcurrent in the line or an overcurrent flowing through the line power converter bridge, using a short-circuit thyristor. Thus, the voltage of the intermediate circuit can be reduced, so that an overcurrent in the line power converter bridge caused by a line overvoltage can be eliminated.
A disadvantage of this method is that very high currents, which are caused by overvoltage in the line and which can be considered an ideal current source, flow through the short-circuit thyristor. If an overvoltage is permanently present in the line, a current causing overheating and a fire hazard will permanently flow through the short-circuit thyristor. Furthermore, this causes an overload in the line power converter bridge, since, in addition to the current needed in the power inverter for the load, the current flowing through the short-circuit thyristor must also be rectified.
Power-recovering converters with DC voltage intermediate circuits convert a polyphase (in particular, three-phase) supply voltage having a fixed frequency into a constant intermediate circuit DC voltage using semiconductor switches and commutation reactors, and then into a three-phase supply voltage with adjustable frequency for the motor to be controlled, using a power inverter. A problem is that in the event of overvoltage in the voltage supply, the DC voltage of the intermediate circuit is increased in an unacceptable manner due to the conversion of the supply voltage by the free-wheeling diodes. In this case, the semiconductor switches, which are controlled to set the constant intermediate circuit voltage, feed the intermediate circuit overvoltage back into the voltage supply system, which may cause unacceptably high currents and subsequent damage.